This updates the SBCL build and release process to be more compatible
with distributed development -- to facilitate moving the upstream
repository into Git.

A detailed description of what is going on here is in
doc/GIT-WORKFLOW.md.

Some highlights:

* Drop version.lisp-expr and branch-version.lisp-expr.

* Auto-generate the version at build time using information
from Git, incorporating:
- Last release number.
- Number of commits on origin/master since last release.
- Current branch, if there are commits not on origin/master.
- Number of commits not on origin/master.
- SHA1 id of the last commit.
- Optional -dirty marker.

* Update release.sh to work with Git.

* Make source-distribution.sh exclude the .git directory from tarballs.

The script automates many of the tasks involved in performing the minimum
level of sanity checking that I am comfortable with, and enumerates others
that remain unscriptable despite 10 years of user interface "improvements"
at Sourceforge. It is unlikely to generalise to other release managers'
setups and taste without a little effort.